Like OrchardEnd says, people 'feeding the ducks' with bread, are actually feeding more rats, and making the ducks sick too! Even though the ducks seem to want more bread, it's like us eating chocolate. We want more, but get sick afterwards!
I've seen rats on the common many times, always near the duck ponds. If we heed the notices and don't feed bread to the ducks, we would be curbing the rat population on our common. (The ducks love duck food though, and it's good for them too! Most pet shops have them)